Currently, on-line monitoring is widely applied to monitor power lines. As explained in U.S. Pat. No. 8,184,015, continuous monitoring of electrical power lines, in particular high-voltage overhead lines, is essential to timely detect anomalous conditions which could lead to a power outage. Measurement of power line spans between successive supports to determine whether the sag (clearance resp.) is lower (higher resp.) than an acceptable maximum (minimum resp.) value is becoming a mandatory requirement in some countries. Given the importance of power line monitoring, several devices and methods have been proposed to date in order to measure the sag and/or some other relevant parameters, either directly or indirectly related to sag. A number of different methods which perform sag measurement are known in prior art. According to some examples, sag of power lines monitoring can be performed by using models, weather models, measuring using image-processing and detection of a target installed on the conductor, measuring using a conductor replica attached to the tower to catch an assimilated conductor temperature without Joule effect, measuring the surface temperature of the phase conductor, measuring conductor tension, measuring using global positioning systems (GPS), measuring the angle of the conductor at the pole or at a location along span, measuring vibrations of the conductor, etc.
Indirect weather-based and/or model-based methods allow to determine sag from measured and/or simulated weather from models and corresponding estimated conductor temperature and supposed sag conditions. One drawback of these methods is that sag conditions are time changing, uncertain and not well-known in practice. These methods are not always capable of providing a correct picture of the situation. If for instance a snow or ice load appears on a line span, the calculation from weather to sag will be erroneous. Moreover, icing on structures highly depends on the temperature of these structures. In case of power lines, joule effect heats the conductor while weather conditions (mainly wind speed) at conductor location cool the conductor and must be adequately estimated. Due to local topology, this task may be very difficult. As joule heating and weather conditions are sensed by the conductor that determines its thermal equilibrium/temperature, point measurements of weather and in particular wind variables even taken in close proximity of the line remain inaccurate. The above comments and limitations are also valid for monitoring using a conductor replica attached to the tower to catch an assimilated conductor temperature without Joule effect.
Methods measuring the surface temperature of the phase conductor and methods measuring the angle of the conductor at the pole or at a location along span suffer from the same drawbacks: relation from temperature to sag can be uncertain and/or erroneous in case of icing and/or variations due to icing must be identified in sag variations along time using for example a temperature estimation.
Methods measuring sag using image processing and detection of a target installed on the conductor is sensitive to reduction of visibility induced by meteorological conditions, and in particular in case of ice fog and freezing fog.
A method measuring sag using global positioning systems (GPS), as detailed in U.S. Patent Application No. 2014/0064389 A1 does not give any information about ice overload.
U.S. Pat. Nos. 5,235,861 and 5,517,864 detail a power line monitoring using tension sensor measurement located at the pole. A drawback of this method is that relation from tension to sag can be uncertain and/or erroneous in case of icing because of unknown apparent conductor weight and/or variations due to icing must be identified in sag variations along time using for example a temperature estimation. The use of this tension installed/developed primarily for real-time ampacity rating also serves for observations of ice on conductors. As detailed in literature [2], the ice accretion can be determined by plots of tension versus net radiation temperature.
U.S. Pat. No. 6,343,515 B1 details a method and apparatus to perform a measure of tension in any location along a wire. Other older patents as for example U.S. Pat. No. 7,424,832 B1, use the same philosophy to measure the tension in a wire. As detailed in the cited patent, a deflection of the cable applied through the sensor produces a tangential force proportional to mechanical tension in the wire/cable. The deformation produced by the force is measured by an integrated strain gauge sensor. This sensor (output from strain gauge) must be calibrated to specific wire size and type. Commercial sensors, of which not-limitative illustrations are given in ref. [5] to [8], based on the corresponding method and apparatus detailed in the cited patents, are also available but these sensors are not directly usable in the domain of power lines domain since no attention was paid therein to high voltage constraints, as for example need for electromagnetic shielding of the monitoring electronics, anti-corona design, etc. Regarding ice monitoring of power lines, this tension-based method suffers from previously mentioned drawbacks as the fact that relation from tension to sag can be uncertain and/or erroneous in case of icing because of unknown apparent conductor weight, and/or variations due to icing must be identified in sag (tension) variations along time using for example a temperature estimation.
In another example, sag of power lines monitoring can be performed by measuring vibrations of the conductor as detailed in U.S. Pat. No. 8,184,015. As detailed in the above-mentioned patent, sag is solely determined by the fundamental frequency estimation of the vibrations of the conductor.
Except for vibrations-based sag monitoring detailed is U.S. Pat. No. 8,184,015, for which no external data such as topological data, conductor (and in particular apparent weight per unit length as explained further) or span data, weather data, or sagging conditions, etc., are needed for sag monitoring, all other methods exhibit at least some limitations in sag monitoring in case of ice loading periods as ice/snow or other accretion that build up on the conductor will increase the apparent diameter and weight of the conductor. In other words, the relation from measured parameters to sag is not known in case of atmospheric accretion. As a consequence, the previous methods, except in case of the vibrations-based method, can lead to errors in sag measurement due to atmospheric accretion and these methods eventually must be coupled to weather measurements/information to give data about ice-snow accretion conditions and/or conductor temperature (as conductor temperature is around 0° C. in case of accretion) but there is no information about ice accretion. The needed weather conditions coming from models and/or weather stations may not be a good estimate of the real weather conditions at conductor location or height, and estimated conductor temperature along the line could be erroneous.
Previously, sag monitoring systems were directly related to dynamic line rating of power lines. Sag increase can be due to various factors, such as previously mentioned: change on current load, change on weather conditions, etc. In order to distinguish between cases of increased sag for the line caused on the one hand by snow/ice loads and on the other hand by high line temperature due to current load and/or weather change, sag increase measurement is in practice set in connection with a conductor temperature measurement and/or a conductor temperature estimation since snow and/or ice load can occur at temperature around 0° C. Some functions have been built in to set threshold values for line temperature beforehand.
However, methods and systems which are not directly linked to dynamic line rating of power lines, and intended for measuring/detecting the potential ice thickness exist.
As explained in ref. [1], traditional methods of estimating icing conditions include video surveillance, non-contact infrared measurement, and temperature sensing at the line surface or core. As explained in ref. [1] as well, Bragg grating (FBG) sensing has exhibited a great potential in transmission line monitoring. Ref. [1] shows that icing monitoring can be achieved using FBG sensors. A drawback of such methods however is that the camera must be power supplied close to the power line. This method is sensitive to reduction of the visibility induced by meteorological conditions while indirect temperature-based and FBG methods is only a part of the solution. Indirect measurements are inaccurate since sag has to be deduced by algorithms which depend on unavailable and/or uncertain data (e.g. ice, wind loading) and/or uncertain models. Sag cannot thus be accurately determined by temperature or indirect measurements in case of atmospheric icing.
As explained for instance in ref. [3] and [9], icing and the thickness of the ice layer on an overhead power line can be determined by measuring inter-electrode capacitance formed by two electrodes mounted on the surface of the conductor by exploiting the time signals and differences in the respective permittivities of air, ice and water. This is only a part of the monitoring of the power lines since sag cannot be determined.
Other methods can be found in prior art. For instance the ice load surveillance sensor IceMonitor™ (detailed in ref. [4]) measures ice growth deposit on structures. A drawback of this system is that the sensor is not taking into account the current in the power line and the real conductor temperature which can be dramatically different from that of the above-mentioned sensor. Indeed, icing appears at conductor temperature around 0° C. and a small amount of current can lead to conductor temperature above the threshold of icing. Sag is not monitored at all and this specific ice monitor is only a part of the solution.
Most of the proposed methods measure some related parameters, which are then used to indirectly compute the overhead conductor sag. Sag variations due to icing must be identified by using a coupled temperature estimation since sag variations can be due to icing and/or change in conductor temperature (at high current for example).